Adding transparencies in blender grease pencil
so I think the reason this has occurred is your multiple pass shading has both overlapped and not quite joined when you've drawn it. Easiest fix is to draw the shading in 1 pass if you can.
If that's not possible you could enter Edit mode and, ensuring your viewport is set to &quot;Select only points&quot; (This is the first button to the right of the Mode drop down box) move the overlapping/miss matching points as desired.
Another solution would be to draw the boundary of the shading with a stroke on a layer and then fill on the multiply layer, hiding the original stroke layer once complete.
Or, my personal preference would be to remove the multiply effect all together and instead go for a black or grey dark layer with a low opacity. This would avoid the compounding colour (making areas darker) but obviously wouldn't fix the gaps.
Good Luck. Shout if you need any further guidance.